1. Field of the Invention
The present invention relates to radio wave receivers with an antenna structure.
2. Background Art
Radio-controlled watches are known which receive the standard waves including time information and set the current time thereof automatically. In these watches, many of the antenna structures which receive the standard waves include a core of magnetic material such as amorphous alloy or ferrite excellent in reception sensitivity and a coil wound around the core.
The reception sensitivity of the antenna structure increases as the core is longer and its radio wave reception area increases. However, since a small electronic device such as a radio-controlled wristwatch has a limited inner space, the antenna structure to be incorporated in the space is required to be small in size.
JP 2007-184894 discloses a radio-controlled wristwatch in which a pair of amorphous films are each disposed at a respective one of ends of a rod-like core provided within a metal case so as to be magnetically coupled to the core to improve the reception sensitivity thereof even when the antenna structure is made small.
When the radio wave receiver is a wristwatch, its case is often made of a metal material such as titanium or stainless steel from the standpoint of designability and a sense of high quality.
When the antenna structure is received within the metal case, a flow of radio waves or magnetic flux is intercepted by the case and eddy currents will be produced in the case, thereby producing no sufficient reception sensitivity.
JP 2007-170991 discloses the use of a magnetic member of low conductivity and high permeability fitted into a cavity provided within a metal case along the antenna structure in order to prevent production of eddy currents in the case, thereby preventing a reduction in the reception sensitivity.
With the invention of JP 2007-184894, the pair of magnetic members of high permeability and low conductivity disposed in the case serve to reduce eddy currents occurring in the metal case, but cannot improve the reception sensitivity. Thus, when the antenna structure is made small in size, it cannot ensure satisfactory reception sensitivity.
It is therefore an object of the present invention to provide reduced-sized antenna structure which captures the magnetic flux of the radio waves efficiently and improves the reception sensitivity, and a reduced-sized radio wave receiver including the antenna structure therein.